The stiffer the boot the more technical the intended end use. Flexible footwear carries greater risk of sprains and strains when you’re carrying heavy packs on varied and difficult terrain with lots of ascent and descent; greater weight requires greater impact protection and denser, more durable cushioning; and crampon use requires a firm sole unit in order to kick steps and front point in soft snow and ice. Expert boot manufacturers tend to categorise their products in a way that clearly indicates what they’re designed for but in general - the greater the distance, the more weight you intend to carry and the more varied (and difficult) the terrain - the more support you’ll require and the stiffer the boot. Many of us develop a personal preference through experience but if you’re lacking this, seek advice and always try on a selection.